In field emission devices, emitters are not heated as with conventional thermionic emission devices, but instead electrons are discharged by applying a strong field to the emitters. Recently research and development are being made into Field Emission Displays (FEDs) and Cathode Ray Tubes (CRTs) which use such field emission devices as a source of electron emission.
The following explains the main body and the driving circuit of a field emission device with reference to FIG. 10.
As shown in FIG. 10, a cathode 102 is formed in a thin film on one surface of a cathode substrate. An emitter 105 and an insulating layer 103 are formed on the cathode 102, and an extraction electrode 104 is in the insulating layer 103. A gate hole is formed in the extraction electrode 104 so as to expose the emitter 105.
Next, an anode 107 is formed on the surface of an anode substrate 106 that faces the cathode substrate 101.
A vacuum of approximately 10−6 Pa is generally maintained in the space between the emitter 104 and the anode 107.
The driving circuit is composed of a driving power source 109 which is connected to the extraction electrode 104, and an acceleration power source 110 which is connected to the anode 107. The cathode 102 is grounded.
The driving circuit applies a driving voltage Vex between the extraction electrode 104 and the emitter 105 in order to generate a field in the area surrounding the emitter 105, and an acceleration voltage Va between the anode 107 and the emitter 105 in order to accelerate electron emission.
FIG. 11 shows the relationship in the above-described field emission device between the driving voltage Vex and the amount of electrons emitted (hereinafter “emission current”) I from the emitter 105.
The figure shows that emission of the emission current I starts when a driving voltage Vex, which is a threshold voltage Vth or higher, is applied to the extraction electrode (a point 1200 in the figure). The emission current I increases according to the solid curved line as the driving voltage Vex is increased.
When the emission current I is set to Ie, the initial operation point of the driving circuit is a point 1201 where the driving voltage Vex is V0 and the emission current I is Ie.
However, the emission current I drops as driving time t elapses, even if the driving voltage Vex is sustained at V0. As shown by the arrow, the solid curved line which shows the relationship between the driving voltage Vex and the emission current I moves to the right as the driving time t elapses. The result after a driving time t1 (for example approximately 5000 hours) is a relationship shown by the broken line. At the point where the time t1 has elapsed, the emission current I is If (a point 1202). The emission current I continues to drop as driving time t elapses.
FIG. 12 shows such a characteristic of a field emission device with driving time t on the horizontal axis and the emission current I on the vertical axis.
As described above, the emission current I drops as driving time t elapses from the initial operation point 1301, and is If after the time t1 has elapsed (a point 1302). After this point the emission current I continues to drop as driving time t elapses.
Furthermore, the emission current I is accompanied by constant low-amplitude fluctuations during driving. These fluctuations are thought to occur because the amount of electrons emitted is made unstable by a small amount of gas that remains in the electron emission space.
As described above, it is difficult to apply field emission devices whose emission current I is unstable to image display apparatuses and various other electronic apparatuses. For example, if such a field emission device is used in a color CRT, the drop and fluctuations in the emission current I cause flickering and degradation in luminosity and color fidelity.
In response to such problems Japanese Laid-open Patent Application No. H9-63466 and Japanese Laid-open Patent Application No. H8-87957 disclose techniques for stabilizing the emission current I by adding a field effect transistor (hereinafter “FET”) function to the device.
However, these techniques have an effect of stabilizing the emission current I up to a certain time after the initial driving, but fail to stabilize the emission current I when the performance in emitting electrons from the emitter deteriorates beyond a certain range as driving time elapses.